1. Field of the Invention
The present invention relates to a semiconductor laser element which has at least one protection layer on at least one end facet.
2. Description of the Related Art
One of the factors which impede increase in maximum output power of semiconductor laser elements is degradation caused by non-radiative current which is generated in vicinities of end facets of resonators when output power is high. Specifically, current which does not contribute to light emission is generated in the vicinities of the semiconductor end facets, and heat is generated at the semiconductor end facets. Therefore, facet degradation occurs due to the heat generation, and the maximum optical output power of the semiconductor laser elements is reduced. In addition, when protection layers or reflectance control layers are formed at the end facets, oxygen is captured by the semiconductor surfaces, and the characteristics of the protection layers or reflectance control layers are degraded by the oxygen. Thus, the maximum optical output power is reduced.
In order to solve the above problems, U.S. Pat. No. 4,656,638 discloses a technique in which a metal layer having a thickness of 10 nm or less is formed on a light-emission end facet so as to passivate near-edge portions of semiconductor layers, and a reflectance control layer is formed on the metal layer, where the metal layer is made of, for example, Al, Si, Ta, V, Sb, Mn, Cr, or Ti, and the reflectance control layer is made of an oxide such as Al2O3. According to the technique disclosed in U.S. Pat. No. 4,656,638, the semiconductor surface is passivated by the external diffusion of oxygen from the semiconductor layers to the metal layer. However, the metal layer is oxidized with the oxygen, and therefore the characteristics and reliability of the semiconductor laser element deteriorate. In order to solve this problem, U.S. Pat. No. 6,323,052 discloses formation of a passivated layer such as a Si layer after removal of oxide from a semiconductor surface. In addition, U.S. Pat. No. 5,144,634 corresponding to Canadian Patent Laid-open No. 2018501 discloses a technique in which a passivation layer made of Si, Ge, or Sb is formed after removal of an oxidation layer at an end facet of a resonator, and a protection layer made of Si3N4, which exhibits high thermal conductivity, is formed on the passivation layer.
Until now, many attempts have been made to form an oxidation layer after formation of a first layer which does not contain oxygen or nitrogen on an end facet of a semiconductor laser element, and many reports have been made on the constituents of the oxidation layer. However, when a reflectance control layer containing an oxide or nitride is formed on the above first layer, the reflectance control layer can be separated from the first layer by exfoliation caused by a difference in the linear expansion coefficient between the layers when heat is generated during laser oscillation. The above separation by the exfoliation becomes prominent, in particular, in a high output power operation. Therefore, sometimes the laser oscillation can suddenly stop.
It is known that the above separation by the exfoliation can be prevented by oxidizing the surface of the first layer not containing oxygen or nitrogen so as to form a thin oxide layer, and forming on the thin oxide layer a reflectance control layer made of an oxide. In addition, in the case where the reflectance control layer is made of a nitride, the above separation by the exfoliation can be prevented by nitriding the surface of the first layer so as to form a thin nitride layer. However, when the thickness of the above oxide layer or nitride layer is too great, the function of the first layer for strengthening the end facet is lost, and the characteristics of the semiconductor laser element deteriorate.